1. Field of the Invention
The present invention relates to a semiconductor element having a parallel p-n junction layer with an arrangement of alternately joining a plurality of drift regions made up of a first conduction type semiconductor and a plurality of partition regions made up of a second conduction type semiconductor. The drift regions extend from a first principal surface side of a semiconductor substrate toward a second principal surface side thereof, the partition regions extend in the same way as the drift regions, and both regions are joined in a direction crossing the direction in which the regions extend. The parallel p-n junction layer becomes a drift layer that allows a current to flow when the semiconductor element is in a turned-on state and becomes depleted when in a turned-off state. The invention particularly relates to a MOSFET (Insulated-gate field effect transistor), an IGBT (Insulated-gate bipolar transistor) and a semiconductor which is a applicable to a bipolar transistor etc., and can be provided with compatibility between a high breakdown voltage capability and a high current capacity capability.
2. Description of the Related Art
Semiconductor elements may be generally classified into lateral elements and vertical elements. A lateral element is provided with electrodes on one of surfaces of a semiconductor substrate to allow a current to flow in a direction in parallel with a principal surface. A vertical element is provided with electrodes on both surfaces of a semiconductor substrate to allow a current to flow in a direction perpendicular to the principal surface. In the vertical semiconductor element, a direction in which a drift current flows when the element is made turned-on is the same as a direction in which a depletion layer is extended by a reverse bias voltage applied when the element is made turned-off. For example, in an ordinary planer n-channel vertical MOSFET, a section of an nxe2x88x92-drift layer with high resistance operates as a region of allowing a drift current to flow in the vertical direction when the MOSFET is in a turned-on state and becomes a depletion region when the MOSFET is in a turned-off state to increase the breakdown voltage.
To shorten a current path in the nxe2x88x92-drift layer with high resistance is to lower drift resistance to a current. This leads to an effect of reducing substantial on-resistance of the MOSFET. However, the expanding width of a depletion layer between a drain and a base, traveling from the p-n junction between a p-base region and the nxe2x88x92-drift region, is adversely narrow. This makes electric field strength in the depletion layer quickly reach the critical electric field strength of silicon to reduce the breakdown voltage. Conversely, in a semiconductor device with a high breakdown voltage, the nxe2x88x92-drift layer is provided as being thick, by which on-resistance is inevitably made increased to result in increased loss. That is, there is a tradeoff between the on-resistance and the breakdown voltage.
It is known that the same tradeoff holds also for such semiconductor elements as IGBTs, bipolar transistors and diodes. Moreover, the problem is common to a lateral semiconductor element in which the flowing direction of a drift current when the element is turned-on differs from the travelling direction of a depletion layer expanded by a reverse bias voltage applied when the element is turned-off. As measures for solving the problem, structures of semiconductor devices are disclosed in, for example, EP-B 0 053 854, U.S. Pat. No. 5,216,275, U.S. Pat. No. 5,438,215 and JP-A-9-266311. In each of the disclosed structures, a drift layer is arranged with a parallel p-n junction layer in which highly doped n-type regions and p-type regions are alternately disposed. The parallel p-n junction layer becomes a depletion layer when the device is in a turned-off state so as to bear a voltage to withstand.
The structural difference between the above semiconductor device and an ordinary planer n-channel vertical MOSFET is that the drift layer is not made up of a layer with a uniform and single conduction type, but of the above-described parallel p-n junction layer. In the parallel p-n junction layer, when the element is in a turned-off state, from each of the p-n junction aligned in the vertical direction of the parallel p-n structure, a depletion layer expands in the lateral direction on both sides of the p-n junction even though an impurity concentration is high. This brings the whole drift region to become a depletion region to allow the device to have a high breakdown voltage. In the specification, the semiconductor element provided with a drift section with such a parallel p-n junction structure is to be referred to as a super junction semiconductor element.
Incidentally, on-resistance of a planer super junction MOSFET (Ronxc2x7A) is generally expressed approximately by the following expression (1), where resistance of a source layer is denoted by Rs, channel resistance is denoted by Rch, resistance of an accumulation layer is denoted by Racc, resistance by a junction FET (JFET) effect is denoted by RJFET, drift resistance is denoted by Rdrift, resistance of a drain layer is denoted by Rd and an area of a region causing the on-resistance is denoted by A:
Ronxc2x7A=(Rs+Rch+Racc+RJFET+Rdrift+Rd)xc2x7A.xe2x80x83xe2x80x83(1)
In the super junction semiconductor element, the drift resistance Rdrift is given by the following expression (2). Therefore, even though a breakdown voltage is increased, only the drift resistance increases in proportion to the breakdown voltage. This allows dramatic reduction in the on-resistance compared with related MOSFETs. Furthermore, for the same breakdown voltage, by reducing a width d of the n-drift region in the parallel p-n junction layer, the on-resistance can be further reduced. In the expression (2), xcexc is a mobility of electrons, xcex5o is the permittivity in vacuum, xcex5s is the specific permittivity of silicon, Ec is critical electric field strength and Vb is a breakdown voltage:
Rdriftxc2x7A=(4xc2x7dxc2x7vb)/(xcexcxc2x7xcex5oxc2x7xcex5sxc2x7Ec2)xe2x80x83xe2x80x83(2)
However, while the drift resistance Rdrift is dramatically reduced, resistance components other than the drift resistance in the expression (1) become significant. In particular, the proportion of the resistance RJFET in the JFET effect is large in the on-resistance. For improving this, application of a so-called trench MOSFET is proposed in which a gate electrode fills each of trenches dug from the substrate surface for inducing a channel in a section on the side wall of the trench. About a trench super junction semiconductor element, there is a disclosure in, for example, JP-A-2002-76339.
However, also in the case of the trench MOSFET, a voltage withstanding structure section is provided in the same way as in the planer MOSFET. Therefore, when the MOSFET has stripe-like trenches, an end portion of each of the trenches is sometimes formed in a region where the structure of the region changes to that of the voltage withstanding structure section. In such a case, the end portion of the trench formed in a shape of a three-dimensional curved surface causes electric field concentration in a region at the end portion of the trench to bring about possible reduction in a breakdown voltage.
In addition, in a transition stage of being shifted from a turned-on state to a turned-off state, the depletion layer is quickly expanded in the parallel p-n junction structure. This prevents accumulated carriers from escaping to cause the discharged carriers to encounter a strong electric field due to electric field concentration, which makes the carriers easily injected into a gate insulator film as hot carriers. Thus, the gate insulator film is degraded to cause such possible lowering in reliability of the gate insulator film as to bring about reduction in a threshold voltage. The applicant discloses in JP-A-2001-313391 a structure of a planer super junction semiconductor device that can suppress injection of hot carriers into a gate insulator layer. However, it is necessary also for the trench super junction semiconductor element to suppress injection of hot carriers into the gate insulator film.
The invention has been made in view of the foregoing with an object of providing a trench super junction semiconductor element which inhibits reduction in a breakdown voltage, and along with this, has a high reliability of the gate insulator film.
In order to achieve the above object, the super junction semiconductor element according to the invention has a low resistance layer and a parallel p-n junction layer between a first principal surface and a second principal surface of a semiconductor substrate. The parallel p-n junction layer includes a plurality of first conduction type drift regions and a plurality of second conduction type partition regions. Both of the first conduction type drift regions and the second conduction type partition regions extend in the vertical direction from the first principal surface side toward the second principal surface side. The parallel p-n junction layer has a structure in which the first conduction type drift regions and the second conduction type partition regions are alternately joined in a lateral direction. Moreover, each of the second conduction type partition regions as a part of a plurality of the second conduction type partition regions has a section with an impurity concentration higher than that on the second principal surface side or a section with a width larger than that on the second principal surface side. The sections each with the higher impurity concentration or with the larger width in the second conduction type partition regions as a part have end portions of the trenches therein that are formed on the first principal surface side.
On the inner surface of each of the trenches, there is provided a gate insulator film. Inside of the trench, covered with the gate insulator film, is filled in with a gate electrode. A second conduction type base region is provided on a surface layer on the first principal surface side so as to be in contact with at least a part of a section along a side wall of each trench. In the second conduction type base region, there are provided first conduction type source regions so that each of them is separated from the first conduction type drift regions by the second conduction type base region and is in contact with a section of the gate insulator film along the side wall of each trench.
In the invention, the section with a higher impurity concentration or with a larger width surrounding the end portion of the trench in the second conduction type partition region with the end portion of the trench formed therein can be disposed in the second conduction type partition region in the parallel p-n junction layer disposed in a region in which a current is allowed to flow in a turned-on state. Moreover, all of the first conduction type drift region, the second conduction type partition region and the trench are stripe-like, and the first conduction type drift region and the second conduction type partition region can be approximately perpendicular to the trench. In this case, in addition to the trench approximately perpendicular to the first conduction type drift region and the second conduction type partition region, a second trench perpendicular to the trench can be provided. Both kinds of the trenches can be connected with each other so as to surround a region in which a current is allowed to flow in the turned-on state. The same is for the case in which the first conduction type drift region and the second conduction type partition region are approximately in parallel with the trench. In this case, a structure can be provided in which the trench and a second trench disposed perpendicularly thereto are connected with each other so as to surround a region in which a current is allowed to flow in the turned-on state.
According to the invention, a part in a shape of a three-dimensional curved surface of the end portion of the trench is surrounded by the section with a higher impurity concentration or with a larger width in the second conduction type partition region in the parallel p-n junction layer. This increases electric field strength at a boundary between the section with a higher impurity concentration or with a larger width in the second conduction type partition region and the first conduction type drift region. Therefore, concentration of electric field strength to the part in the shape of the three-dimensional curved surface of the end portion of the trench is lessened.